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Figure 1<br />

The coagulation cascade.<br />

Page 266<br />

receptor. Thrombin generation is itself, however, the result of Factor Xa in complex with Factor Va on a<br />

phospholipid surface (the prothrombinase complex) acting on prothrombin. Vascular injury is the<br />

initiating event in the coagulation process, causing the activation of Factor Xa <strong>by</strong> the Factor VIIa/tissue<br />

factor complex. Factor Xa is, therefore, a central and crucial enzyme directly leading to the production<br />

of thrombin and its inhibition should be effective in blocking thrombogenesis. As a consequence of its<br />

key role early in the coagulation cascade process Factor Xa represents a potentially valuable therapeutic<br />

target for potent and specific inhibition.<br />

III. Proof of Principle for a Factor Xa Inhibitor<br />

In recent years the method <strong>by</strong> which certain hematophageous organisms maintain blood flow during<br />

feeding has been determined. Interestly, several of these organisms utilize Factor Xa inhibitors to<br />

prevent coagulation [13–15]; the tick anticoagulant peptide (TAP), a small protein isolated from the<br />

Ornithidoros moubata tick [13], and antistasin isolated from the Haementeria officinalis leech [14] are<br />

both potent and selective inhibitors of Factor Xa. As expected, these molecules are effective<br />

antithrombotics in several animal models of thrombosis (Table 1) and provide an important proof of<br />

principle with regard to the potential effectiveness of Factor Xa inhibitors as therapeutic anticoagulants.<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_266.html [4/5/2004 5:11:49 PM]

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